Cousins, the Flyers' first shooter, skated in slowly and slid a low shot between Washington goaltender Braden Holtby's pads.
Kuznetsov came in slowly and faked Mason to the ice, but the goaltender was able to lunge forward and glove his attempt.
"You saw how slowly they were coming in," Mason said. "I think the idea there is to make sure the goalie loses his depth, and I tried to stay patient. The second one I didn't even come out to get set until he was past the top of the circles because of how slow he was coming in. [I] just tried to wait him out, and tonight I was fortunate. Just cover as much surface area as you can and take the angle, the trajectory of the shot and fortunately it went in the glove."
Gagner won it when he skated in on Holtby and snapped a wrist shot between his pads.
"It felt good," Giroux said. "It was a physical game. They're a good team on the other side. We battled hard and got only that one goal. They're one of the best teams we've played in a while here. It was fun to compete with them."